instant of initial delamination, where its peak is the cohesive strength. (H) The computed peel force as a function of time for an EES adhesive with a triangular pattern of small holes (diameter D = 200 mm) on the skin. The hole area fraction is a ¼ ffiffiffi 3 p pD2=6L2. (I) The computed peel force as a function of time for triangular and square patterns of large holes (diameter D = 1 mm) with the hole area fraction a = 35%, where a ¼ pD2=4L2 and a ¼ ffiffiffi 3 p pD2=6L2 for square and triangular patterns, respectively. RESEARCH | RESEARCH ARTICLE Downloaded from https://www.science.org on July 15, 2022 interfaced to a microcontroller in a Bluetooth Low Energy (BLE) system configured with the customized circular buffer decoding routine (fig. S14). The primary antenna connects to the host system for simultaneous transfer of RF power to the ECG EES and the PPG EES. Operation is possible at vertical distances of up to 25 cm, through biological tissues, bedding, blankets, padded mattresses, wires, sensors, and other materials found in NICU incubators, for full-coverage wireless operation in a typical scenario (fig. S15). BLE radio transmission then allows transfer of data to a personal computer, tablet computer, or smartphone with a range of up to 20 m. Connections to central monitoring Chung et al., Science 363, eaau0780 (2019) 1 March 2019 4 of 12 A C Mesh Electrode Solid Electrode Commercial Electrode Mesh Electrode Solid Electrode Commercial Electrode B D T/m 0.1 0 In-plane gradient of magnetic field density Out-of-plane gradient of magnetic field density T/m 0.1 0 E F G H I J 0 5 10 15 20 0.00 0.03 0.06 0.09 0.12 Location (mm) cit enga mf ot nei dar G ) m/ T( yti sned dl eif Commercial Electrode Solid Electrode Mesh Electrode T/m 0.3 0 T/m 0.3 0 1.5T 100 200 300 400 -25 -20 -15 -10 -5 0 Frequency (MHz) S11 (dB) 3T 7T 9.4T Time (s) mu mi xa M ( egnah C er ut ar ep meT °C) Device (Cu Layer) Skin On Off 1E-4 0.001 0.01 0.1 1 10 0.0 0.3 0.6 0.9 1.2 0 5 10 15 20 0.00 0.03 0.06 0.09 0.12 Location (mm) Gradient of magnetic field density (T/m) Commercial Electrode Solid Electrode Mesh Electrode Temperature ( °C) ECG EES mounted (coil) Bare Phantom Skin Off On Off 0 7 14 21 28 35 42 49 56 20.2 20.3 20.4 20.5 20.6 20.7 Time (min) 0 10 20 30 40 20.3 20.4 20.5 20.6 20.7 20.8 Time (min) Temperature ( °C) ECG EES mounted (mid) Bare Phantom Skin Off On Off Fig. 3. Theoretical and experimental aspects of radiolucency. (A) Computational results for the distributions of the in-plane gradient of the magnetic field density associated with a mesh electrode as in Fig. 1A (left), a solid electrode (no mesh; center), and a commercial NICU electrode (right) for conditions associated with an MRI scan at 128 MHz. (B) Calculated in-plane gradients of the magnetic field density associated with a complete ECG EES at 128 MHz. (C) Distributions of the out-ofplane gradient of the magnetic field density associated with a mesh electrode, a solid electrode, and a commercial NICU electrode for conditions associated with an MRI scan at 128 MHz. (D) The out-of-plane gradients of magnetic field density induced on the ECG EES at 128 MHz. (E) The in-plane gradient of the magnetic field density evaluated along the horizontal dashed lines in (A). (F) The out-of-plane gradient of the magnetic field density along the horizontal dashed lines in (C). (G) S11 parameter of the ECG EEG as a function of frequency. The vertical dashed lines indicate operating frequencies of 1.5-T, 3-T, 7-T, and 9.4-T MRI scanners at 64 MHz, 128 MHz, 298 MHz, and 400 MHz, respectively. (H) Computational results for the maximum change in temperature of an ECG EES on skin during an MRI scan. (I) Temperature changes collected using two fiber-optic thermometers located at the interface between an ECG EES (at the loop antenna, coil) and a piece of phantom skin (blue) and on the surface of the phantom skin (red) during MRI scanning (3-T MRI). (J) Temperature changes collected by two fiber-optic thermometers at the interface between an ECG EES (at one of the mesh electrodes) and a piece of phantom skin (blue) and on the surface of the phantom skin (red) during MRI scanning (3-T MRI). RESEARCH | RESEARCH ARTICLE Downloaded from https://www.science.org on July 15, 2022 systems in the hospital can then be established in a straightforward manner. Low-modulus mechanics, soft interface adhesion, and implications for neonatal skin safety The essential mechanics of these systems decrease risks for skin injury relative to existing clinical standards. The global incidence of skin breakdown in hospitalized neonates ranges between 31 and 45%, with medical devices and associated adhesives being a major iatrogenic cause (3, 7). Additionally, pressure-related skin injuries occur in 26% of hospitalized infants less than 3 months of age (7) with 80% directly related to medical devices, where PPG modules are the most common culprit (18). By age 7, more than 90% of children born preterm (<30 weeks gestation) and previously cared for in the NICU exhibit residual scars secondary to monitoring probes, adhesives, and invasive medical interventions (4). Premature neonates are particularly high-risk given that their epidermis and dermis are only 40 to 60% as thick as adult skin, with incomplete cornification, decreased mechanical strength, and greater propensity to scar (19). Although all neonatal skin is susceptible to iatrogenic injury, premature neonates are especially vulnerable. At 24 to 30 weeks gestation, the epidermis is 60% as thick as it is at 36 to 40 weeks (20), and it is considerably more fragile. As a result, removal of adhesives necessary for securing medical equipment poses a greater risk with greater prematurity, where up to 15% of a neonate’s total skin surface area can be traumatized daily (21). The inherently thin, soft mechanical properties of the sensors (Fig. 1A) reported here allow for adhesion via van der Waals forces alone. Effective moduli in the range of 200 to 300 kPa (Fig. 2A) lead to minimal normal and shear stresses at the skin interface associated with natural motions of the neonate. The mechanical decoupling afforded by the microfluidic channel decreases these stresses by up to a factor of 2.5 (Fig. 2B) relative to otherwise similar designs without the microfluidics. Experimental and theoretical studies reveal additional fundamental aspects of the soft mechanics and adhesion in these systems. Simulations that use the cohesive zone model (fig. S16) allow quantitative examination of the physics associated with removal of conventional adhesives (e.g., Argyle Hydrogel Adhesive Baby Tape Strips, Covidien) and EES devices (modeled as an effective medium; Fig. 2, C and D) from surfaces with mechanical properties reflective of neonatal skin. The differences between the magnitudes of deformations induced in the skin, at identical peel forces, are notable (Fig. 2C). The forces at steady-state peeling rates are different by approximately a factor of ~10 (Fig. 2D), with reduction in the maximum von Mises stress on the skin by a factor of 4.3. Experimental testing on adult skin (Fig. 2, E and F) shows similar behavior, including a substantial reduction in peel force (~1000%; fig. S17) of an EES relative to that of a traditional adhesive. Analysis of these experimental results defines the adhesion energy at the interface between the EES and skin: G = 16 N/m. The presence of the microfluidic channel (fig. S18) serves an important role in determining the adhesion properties of the EES, as shown in Fig. 2G. At steady state (>2 s), the peel forces (F) with and without the microfluidics are approximately the same, consistent with a scaling relationship that depends only on G and the width of the device, W, as F = G × W (22). In other words, the adhesion energy defines the steady-state peeling force. At the initiation of peeling, however, in the non–steady-state regime when the forces on the skin are most important, the cohesive strength determines the force. Specifically, the interface starts to delaminate when the normal stress reaches ~20 kPa (Fig. 2G, inset). The microfluidic channel reduces the effective modulus of the EES and, as a consequence, increases the ability of the device to deform under applied force. The consequent reduction in the size of the cohesive zone at the delamination front (fig. S19) decreases the peel force for the same peak stress (cohesive strength). Further reductions can be achieved by the addition of perforations through the open regions of the EES platform, as shown in fig. S20 for different patterns of holes. Figure 2H highlights the peel force, the primary driver of epidermal stripping in fragile neonatal skin (3), as a function of time during peeling for a regular triangular pattern of holes (diameterD= 200